1. Field of the Invention
The present invention relates to an iron silicide optical material which is used for optical interconnections for optical communications, optical sensors, and solar cells. The present invention also relates to an optical element using the optical material.
2. Description of the Related Art
There is growing demand for light-emitting elements and photo-receiving elements using material based on silicon so that these elements can be incorporated in a silicon substrate to be used in an optical sensor and for optical interconnections. A compound semiconductor such as, for example, gallium arsenide may be used as the material for the optical elements on the silicon substrate. However, it is difficult to incorporate such a compound semiconductor in the silicon substrate without causing defects in the structure of the compound semiconductor, and the resulting compound semiconductor exhibits poor thermal stability. Moreover, manufacturing of the compound semiconductor requires specific steps in addition to the conventional steps for manufacturing silicon integrated circuits, resulting in increased manufacturing costs. Accordingly, a technique for making silicon-based light-emitting and photo-receiving structures, which requires the conventional silicon-IC production process only, has been desired.
Conventionally, among the optical elements manufactured by the conventional technique, a light-emitting element containing iron silicide, operating at a suitable wavelength for silica glass optical fibers, which is approximately 1.5 xcexcm, is known as a current-injection element (D. Leong, M. Harry, K. J. Reesen, and K. P. Homewood, xe2x80x9cNATURExe2x80x9d Vol. 387, Jun. 12, 1997, pp. 686-688). This light-emitting element is fabricated by depositing an n-type silicon layer and a p-type silicon layer on a (100) oriented n-type silicon substrate by an epitaxial growth method, implanting iron ions in the p-type silicon layer in the vicinity of the p-n junction interface on the substrate, and annealing so as to form a monocrystalline layer of beta iron silicide (xcex2-FeSi2) having an orthorhombic structure.
However, the above-described optical element containing iron silicide has an external quantum efficiency of approximately 0.1%, which is low and causes a problem. Moreover, the optical element emits a sufficient amount of light at a cryogenic temperature but not at room temperature. Several other studies made in regard to the light-emission characteristics of xcex2-FeSi2 show that the optical element exhibits a long luminescence decay time, i.e., approximately several ten of microseconds. Optical interconnections and optical communications require much shorter luminescent decay time.
In view of the above, the present inventors have proposed a luminescent substance in which semiconductor particles of xcex2-FeSi2, having a particle diameter on the order of nanometers, are dispersed in p-type or n-type amorphous silicon or p-type or n-type amorphous silicon carbide (Japanese Unexamined Patent Application Publication No. 11-340499). Since the xcex2-FeSi2 semiconductor particles having the particle diameter on the order of nanometers are crystallized and are dispersed in the amorphous silicon or in the amorphous silicon carbide having a large bandgap, injected carriers are confined inside the semiconductor particles, thereby enhancing the light-emission efficiency compared to the conventional monocrystalline xcex2-FeSi2.
Among conventional photo-detectors, a variety of solar-cell elements, such as a single crystal silicon type, a polycrystalline silicon type, an amorphous silicon (a-Si) type, and a gallium arsenide (GaAs) type are available on the market. However, these solar-cell elements have problems with regard to their cost and conversion efficiency, and a photo-receiving material having higher efficiency at low cost is desired. The xcex2-FeSi2 has a significantly large optical absorption coefficient at a wide wavelength range for sunlight, and can be manufactured as an ultra thin film. Thus, when the xcex2-FeSi2 is used as a photo-detector of a solar cell, the amount of raw material used can be reduced and the costs can be decreased.
The luminescent substance disclosed in the above-described Japanese Unexamined Patent Application Publication No. 11-340499 can still be improved from the point of view of light-emission efficiency and the efficiency of the manufacturing method.
Moreover, because xcex2-FeSi2 has a carrier mobility which is too small for use as a material for the photo-receiving material of solar cells, xcex2-FeSi2 is yet to be applied to solar cells.
Accordingly, it is an object of the present invention to provide a silicon-based optical material capable of achieving high light-emission efficiency and high photo-receiving efficiency.
Another object of the present invention is to provide an optical material which is capable of achieving a luminescent decay time of several tens of nanoseconds or less and which can be applied to high-speed optical communications.
Still another object of the present invention is to provide an optical material which emits and receives light at room temperature.
Yet another object of the present invention is to provide an optical element using the above-described optical material.
To these ends, the present invention provides an optical material according to an aspect of the invention comprising a crystalline silicon and FexSi2 in the form of dots, islands, or a film. The FexSi2 has a symmetrical monoclinic crystalline structure belonging to the P21/c space group and is synthesized at the surface or in the interior of the crystalline silicon. The monoclinic structure corresponds to a deformed structure of xcex2-FeSi2 generated by heteroepitaxial stress between the {110} plane of the FexSi2 and the {111} plane of the crystalline silicon, wherein the value of x is 0.85xe2x89xa6xxe2x89xa61.1.
Because the xcex2-FeSi2 is artificially deformed and the crystalline structure thereof is changed from an orthorhombic crystal to a monoclinic crystal, the xcex2-FeSi2 becomes less symmetrical. Thus, dipole transitions are allowed between many electronic states, and the oscillator strength which determines characteristics of the light-emitting/photo-receiving material is larger than that of the xcex2-FeSi2.
Preferably, the lattice constant of the c axis of the FexSi2 having the monoclinic crystalline structure is 7.68xc2x10.20 xc3x85, which is equal to the interatomic distance of the {111} plane of the crystalline silicon, the lattice constant of the a axis of the FexSi2 is 10.17xc2x10.35 xc3x85, the lattice constant of the b axis of the FexSi2 is 7.75xc2x10.35 xc3x85, and the angle defined by the a axis and the b axis of the FexSi2 is 95xc2x13xc2x0. This crystalline structure is hereinafter referred to as xe2x80x9cxcex2xe2x80x2-Ixe2x80x9d.
Preferably, the thickness of the FexSi2 having the monoclinic crystalline structure is 5 to 2,000 xc3x85. In this manner, the FexSi2 can maintain the monoclinic crystalline structure. This crystalline structure is hereinafter referred to as xe2x80x9cxcex2xe2x80x2-Ixe2x80x9d.
The optical material may further comprise xcex2-FeSi2 having an orthorhombic crystalline structure. The total thickness of the FexSi2 and the xcex2-FeSi2 is preferably 200 to 10,000 xc3x85.
In this manner, the thickness of the layer can be increased compared to the optical material comprising FexSi2 only and the optical material can be easily manufactured. Also, because of the monoclinic crystals, light-emitting intensity and photo-receiving efficiency can be improved compared to the optical material using only xcex2-FeSi2. When the thickness exceeds 10,000 xc3x85, all the crystals will change to xcex2-FeSi2.
Another aspect of the present invention provides an optical element comprising one of a light-emitting layer and a photo-receiving layer comprising the above-described FexSi2. The crystalline silicon is of p-type or n-type.
In this manner, the optical element having high light-emitting/photo-receiving efficiency and a shorter luminescent decay time can be obtained.
Still another aspect of the present invention provides an optical material comprising a crystalline silicon and FexSi2 in the form of dots, islands, or a film, in which the FexSi2 has a symmetrical monoclinic crystalline structure belonging to the C2/c space group and is synthesized at the surface or in the interior of the crystalline silicon. The monoclinic structure corresponds to a deformed structure of xcex2-FeSi2 generated by heteroepitaxial stress between the {101} plane of the FexSi2 and the {111} plane of a crystalline silicon, wherein x is 0.85xe2x89xa6xxe2x89xa61.1.
Because the xcex2-FeSi2 is artificially deformed and the crystalline structure thereof is changed from an orthorhombic crystal to a monoclinic crystal, the xcex2-FeSi2 becomes less symmetrical. Thus, dipole transition is allowed between electronic states, and oscillator strength which indicates characteristics of light-emitting/photo-receiving material is larger than that of the xcex2-FeSi2.
Preferably, the lattice constant of the b axis of the FexSi2 having a monoclinic crystalline structure is 7.68xc2x10.20 xc3x85, which is equal to the interatomic distance of the {111} plane of the crystalline silicon, the lattice constant of the a axis of the FexSi2 is 10.14xc2x10.35 xc3x85, the lattice constant of the c axis of the FexSi2 is 7.76xc2x10.35 xc3x85, and the angle formed by the a axis and the c axis of the FexSi2 is 95xc2x13xc2x0. This crystalline structure is hereinafter referred to as xe2x80x9cxcex2xe2x80x2-IIxe2x80x9d.
Preferably, the thickness of the FexSi2 having the monoclinic crystalline structure is 5 to 2,000 xc3x85.
In this manner, the FexSi2 can maintain the monoclinic crystalline structure.
The optical material may further include xcex2-FeSi2 having an orthorhombic crystalline structure. The total thickness of the FexSi2 and the xcex2-FeSi2 is preferably 200 to 10,000 xc3x85.
In this manner, the thickness of the layer can be increased compared to the optical material comprising FexSi2 only and the optical material can be easily manufactured. Also, because of the monoclinic crystals, light-emitting intensity and photo-receiving efficiency can be improved compared to the optical material using only xcex2-FeSi2. When the thickness exceeds 10,000 xc3x85, all the crystals will change to xcex2-FeSi2.
Yet another aspect of the present invention provides an optical element comprising one of a light-emitting layer and a photo-receiving layer comprising the above-described FexSi2. The crystalline silicon is one of p-type and n-type.
By using the optical material of the present invention, the optical element having high light-emitting/photo-receiving efficiency and a shorter luminescent decay time.
The optical material of the present invention having such a crystalline structure has the following advantages.
First, since the material constituting the light-emitting/photo-receiving active layer is monoclinic FexSi2 which is less symmetrical than the orthorhombic xcex2-FeSi2, dipole transitions are allowed between many electronic states, and the oscillator strength which determines characteristics of the light-emitting/photo-receiving material is larger than that of the xcex2-FeSi2.
Second, the optical material operates at a wavelength of 1.5 xcexcm which corresponds to the low-loss wavelength of optical fibers.
Third, since the luminescent decay time is several ten nanoseconds or less and is shorter than that of the conventional iron silicide material, the optical material can be applied to high-speed communications.
Fourth, since a layer of the optical material can be deposited on the {111} surface of the crystalline silicon, a light-emitting/photo-receiving active layer can be manufactured at reduced costs.
Fifth, because the monoclinic FexSi2 can be made by an epitaxial growth method, defect density at the interface between the silicon and the FexSi2 is reduced and the resulting optical material is suitable for an current injection element.
Sixth, the optical material is capable of emitting or receiving the light in infrared region of approximately 1.5 xcexcm at room temperature.
Finally, because carrier mobility is enhanced compared to the conventional iron silicide material, rapid and efficient response is possible when light is received.
It should be noted here that, in this specification, the {111} plane, the {110} plane, and the {101} plane are the generic names which include the followings.
{111} is a generic name of (111), ({overscore (1)}11), (1{overscore (1)}1), (11{overscore (1)}), ({overscore (1)}{overscore (1)}1), (1{overscore (1)}{overscore (1)}), and ({overscore (1)}1{overscore (1)}), ({overscore (1)}{overscore (1)}{overscore (1)}).
{110} is a generic name of (110), ({overscore (1)}10), (1{overscore (1)}0), ({overscore (1)}{overscore (1)}0).
{101} is a generic name of (101), ({overscore (1)}01), (10{overscore (1)}) and ({overscore (1)}0{overscore (1)}).